1. Field of the Invention
The present invention relates to air tubes for use with spirometers, and to spirometers using such air tubes. More particularly, the present invention relates to air tubes which are disposable and at least partially biodegradable, and to calibration and tracking techniques for ensuring a high level of accuracy when the disposable air tubes are used with the spirometers.
2. Description of Related Art
Spirometers are devices used to measure the volume and flow rate of gas exhaled and inhaled by a user or patient, for example, a human being. Two general types of spirometers measure volume and flow, respectively. For the flow type, the actual port of the spirometer used to measure flow is the pneumotach of which Fleisch is one type. These measurements are important for physiological studies and for diagnostic analysis of the pulmonary performance of the spirometer user. For example, the effects of various medicines used to treat patients with pulmonary or asthmatic problems can be analyzed by monitoring the volume and flow rate of gas exhaled before and after the administration of medication. Several devices are available on the market which are known as pneumotachs, such as the Fleisch Pneumotach. These devices depend on a laminar air flow past a resistance element. Other spirometers employ more sophisticated electronics so that laminar flow is not needed.
Measuring the pressure difference or differential pressure of exhaled gas across an element which creates or causes the pressure difference is the basis for differential pressure spirometers. In such differential pressure spirometers, it is important that the air tube (pneumotach) be precisely configured and positioned, for example, relative to the pressure sensing and electronics systems of the spirometers so that measurements can be reliably and reproducibly made. Such precisely configured pneumotachs, rather than being disposable, are made out of metals or durable plastics to be long lasting and effective after many uses without structural degradation. See, for example, Waterson et al U.S. Pat. 5,137,026, the disclosure of which is hereby incorporated in its entirety by reference herein.
Since most spirometers involve passing exhaled gas directly from the respiratory system of a user into the instrument for measuring, one important complication of using such devices is contamination from one patient to another patient if the same spirometer is employed by both. Various approaches to overcoming this contamination problem have been suggested. A particularly popular approach is to use a disposable mouthpiece and/or bacterial filter over the inlet to the spirometer. The patient using the spirometer comes in contact only with the mouthpiece and/or bacterial filter and is able, at least in theory, to avoid contaminating the remainder of the device. Drawbacks to this approach include the relative expense of such mouthpieces/filters, and the relative inefficiency of such systems.
Another approach to overcoming this contamination problem is to sterilize in-between patients the portion or portions of the spirometer which come in contact with the user and/or exhaled air. Drawbacks to this approach include having to spend additional capital on sterilization equipment and supplies, having to monitor the operation and efficacy of the sterilization equipment, and having to purchase relatively durable and expensive spirometers to withstand the sterilization procedures.
A third alternative that has been suggested is the use of disposable spirometer components. See, for example, Norlien et al U.S. Pat. No. 5,038,773; Acorn et al U.S. Pat. No. 5,305,762; Karpowicz U.S. Pat. No. Des. 272,184; Boehringer et al U.S. Pat. No. 4,807,641; and Bieganski et al U.S. Pat. No. 4,905,709. Such previous disposable spirometer components have generally been made out of durable plastics or medical grade metals so that, even though they are disposable, the cost of producing such components is relatively high. In addition, such disposable components are relatively difficult to dispose of, for example, because they are made of durable and long lasting materials.
An element of human error can exist to introduce contamination into a spirometer system, even with the use of disposable spirometer components. For example, a user who does not dispose of an air tube after use and, instead, leaves the air tube in the spirometer for subsequent use by another patient, can cause the subsequent patient to be contaminated. The subsequent use of the air tube can also introduce excessive condensation into the air tube, which can result in inaccurate spirometry readings.
The economical manufacture of a relatively inexpensive spirometer component from a low cost and/or biodegradable material has heretofore been prohibitive because of, for example, quality control concerns. General industry specifications require high quality spirometer components but the quality of these components can decrease as the components are made biodegradable, for example, placement of these components within the spirometer can also present problems. The placement of the resistive element within each air tube can affect the performance of the overall spirometer, for example. The resistive element should be placed in a normal or perpendicular configuration relative to the interior wall of the air tube and, further, should be placed at exact, predetermined distances from the two opposing ends of the air tube. Prior art resistive elements often do not exhibit linear resistance-versus-flow-rate responses. More particularly, resistive elements configured to exhibit good resistance at high flow rates often do not perform adequately at low flow rates and, on the other hand, resistive elements configured to perform well at low flow rates often do not provide ideal resistance at high flow rates. Thus, any possibility of manufacturing a relatively inexpensive spirometer, as an alternative to the existing durable plastic or metal non-biodegradable components of the prior art, would appear to be vitiated due to manufacturing and performance concerns. These manufacturing concerns include the inconsistencies between various disposable, biodegradable spirometer components that may be produced on an assembly line and, further, include subsequent performance variances between the spirometer components resulting from these inconsistencies.
Inconsistencies in these components may be augmented when they are assembled together or placed into the spirometer. For example, a throughport of an air tube may not be perfectly formed, and the subsequent placement of this throughport onto the spirometer may introduce abnormally low pressure readings due to air leakage around the pressure port. Even placement of the resistive element within the air tube, as another example, may not be exact between various assemblies and, accordingly, a problem of accuracy may even be prevalent among existing durable plastic or metal non-biodegradable components as well. Accordingly, it would be advantageous to provide a means of ensuring high performance quality and consistency between various spirometer components from an assembly line, regardless of whether the spirometer components are metal, plastic, or biodegradable.